Polymers chain geometries

Figure 6.22 Polymer chain geometries (a) linear (b) branched (c) cross-linked (d) star (e) dendrimer...

Another important factor affecting the electronic properties is the steric barrier to planarity along the polymer chain. Since polyheterocycles and polyarylenes must adopt a planar geometry in the ionized state to form quinoid-like segments, steric factors that limit the ability of the polymer to adopt geometries which are planar with respect to adjacent rings have a detrimental effect on the electronic properties (181). 

Using different elongational flow geometries, the CS transition of a flexible polymer chain as well as the phenomenon of hysteresis for the reversed process of SC transition has been confirmed experimentally by the Bristol group [8, 9], the Cal-Tech group [10, 53] and the Paris group [11, 54]. 

Since the value of (r ) is a property of the polymer only, depending as it does only on chain geometry, it follows that the condition of the polymer at the 6 temperature in different solvents is exactly the same. The polymer behaves as though it were thermodynamically ideal showing no interaction at all with the solvent. 

With these features in mind, we envisioned a new family of macrocyclic ligands for olefin polymerization catalysis (Fig. 9) [131, 132], We utilized macrocycles as the ligand framework and installed the catalytic metal center in the core of the macrocycles. Appropriate intra-annular binding sites are introduced into cyclophane framework that not only match the coordination geometry of a chosen metal but also provide the appropriate electronic donation to metal center. The cyclophane framework would provide a microenvironment to shield the catalytic center from all angles, but leaving two cis coordination sites open in the front one for monomer coordination and the other for the growing polymer chain. This could potentially protect the catalytic center and prevent it from decomposition or vulnerable side reactions. 

M. Grell, D.D.C. Bradley, X. Long, T. Chamberlain, M. Inbasekaran, E.P. Woo, and M. Soliman, Chain geometry, solution aggregation and enhanced dichroism in the liquid-crystalline conjugated polymer poly(9,9-dioctylfluorene), Acta Polym., 49 439-444, 1998. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

In an early attempt to model the dynamics of the chromatin fiber, Ehrlich and Langowski [96] assumed a chain geometry similar to the one used later by Katritch et al. [89] nucleosomes were approximated as spherical beads and the linker DNA as a segmented flexible polymer with Debye-Huckel electrostatics. The interaction between nucleosomes was a steep repulsive Lennard-Jones type potential attractive interactions were not included. 

Now let us examine simple vinyl polymers, with only one site of substitution per repeat unit. When we look at a polymer chain, we focus only on combinations of diads or couples. For our discussion, we will use segments of poly (vinyl chloride). The geometries can be divided into three general groups. The first group, in which the substitutes, or here the chloride atoms, are all identical with adjoining neighbors, comprise meso diads. Polymers or sections of polymers that contain meso diads are referred to as isotactic. 

By definition, polymer brushes are made up of polymer chains grafted (tethered) by one end to a surface or an interface (Fig. 1) [ 1 - 3]. The density can be small or high in the latter case, the polymer chains are crowded and forced to stretch in order to avoid other chains. This results eventually in an equilibrium condition where no external field is necessary to force the chains into this geometry. 

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